Portable air tools having built in transducers are well known in the industry and are often used, for example, as nut runners in monitored production lines. An example of an angle nut runner with built in torque transducer is shown in U.S. Pat. No. 3,858,444.
Nut runners with built in transducers are frequently used in automobile production lines. These air tools with built in transducers are coupled to a centralized computer to monitor and record torque applied to each fastener for each car by serial number. This monitored and recorded information provides a useful data base for quality control and maintenance purposes.
The reliability of this data is dependent upon the transducer circuit being in a balanced condition when unloaded to provide a constant and predictable frame of reference for measuring torque applied. However, the strain gauge elements in the transducer circuit tend to change impedance over use and time. This change in impedance is not necessarily uniform for the strain gauge elements forming the built in transducer resulting in the transducer drifting in value from its original zero or balanced reference condition. This drift or change in the balanced or reference condition of the transducer circuit has been recognized and can be corrected to a certain extent by computer programming compensating for such drift electronically to establish the correct balanced condition to provide reliable and comparative output data. Unfortunately, the extent of drift or change in the respective transducer strain gauge elements may exceed in value the computer system's capacity for accurate correction. In such case, the computer issues an error signal for the transducer of the affected tool requiring that the transducer be corrected or replaced before further use of the tool.
This problem normally requires the tool to be removed from the production line and returned either to the maintenance shop at the plant or to the transducer manufacturer. This remote maintenance on the transducer is ineffecient and expensive.